K\"ahler differentials of extensions of valuation rings and deeply ramified fields

Abstract

Assume that $(L,v)$ is a finite Galois extension of a valued field $(K,v)$. We give an explicit construction of the valuation ring $\mathcal O_L$ of $L$ as an $\mathcal O_K$-algebra, and an explicit description of the module of relative K\"ahler differentials $\Omega_{\mathcal O_L|\mathcal O_K}$ when $L|K$ is a Kummer extension of prime degree or an Artin-Schreier extension, in terms of invariants of the valuation and field extension. The case when this extension has nontrivial defect was solved in a recent paper by the authors with Anna Rzepka. The present paper deals with the complementary (defectless) case. The results are known classically for (rank 1) discrete valuations, but our systematic approach to non-discrete valuations (even of rank 1) is new. Using our results from the prime degree case, we characterize when $\Omega_{\mathcal O_L|\mathcal O_K}=0$ holds for an arbitrary finite Galois extension of valued fields. As an application of these results, we give a simple proof of a theorem of Gabber and Ramero, which characterizes when a valued field is deeply ramified. We further give a simple characterization of deeply ramified fields with residue fields of characteristic $p>0$ in terms of the K\"ahler differentials of Galois extensions of degree $p$.